Very few metals occur in pure form in nature. They are usually present cationically as oxides, sulfides, arsenides, halides, carbonates, phosphonates, sulfates, etc. They occur as metal-comprising minerals or mineral mixtures which are more or less conglomerated with rock and if of economic importance are referred to as ores.
To isolate the pure elements, the minerals and ores have to be processed and the metals extracted therefrom. One possibility is the “leaching process” in which the metal ions are extracted directly from the ore by means of acidic or basic aqueous solutions. The leachant used depends on the type of ore and the occurrence in the respective deposit. In many cases, dilute sulfuric acid (from 1 to 5% by weight) has been found to be useful.
This process can be applied successfully to, for example, low-copper ores and waste products which have a copper content of less than 2% by weight, for example the copper-comprising residues obtained in the burning of pyrite in sulfuric acid manufacture. These starting materials are, if necessary after prior roasting, treated with dilute sulfuric acid and the copper is isolated from the resulting copper sulfate solution by means of further work-up steps.
The usual way of extracting the copper is to make a heap of the copper-comprising material and then spray it from above with dilute sulfuric, so that the dilute sulfuric acid can percolate or trickle through the copper-comprising material. The copper-enriched mother liquor collects at the bottom of the heap and is passed to further work-up.
However, the dilute sulfuric acid has to act on the copper for a period of up to a number of weeks in order to dissolve satisfactory amounts of metal ions from the ore. In general, the yields in copper winning by this process are low.
A process for the acid digestion of copper ores having a low copper content is described in U.S. Pat. No. 4,120,935. Here, the copper ore is wetted with concentrated sulfuric acid and is left for at least 3 days before the ore is washed with from 5 to 10 times the amount of low-concentration sulfuric acid.
German application DE 10 2004 063 500.5 describes the addition of surfactants to the dilute sulfuric acid to increase the yield. The surfactants are selected so that in a concentration of from 0.01 to 0.3% by weight in a 2% strength aqueous H2SO4 solution at 23° C. they lead to a reduction in the contact angle on glass by at least 10° after 1 second.
EP 0 536 914 describes the use of fluoroaliphatic surfactants of the general formula (Rf)n(Q)xZ, where Rf is a fluoroaliphatic radical, Q is a linking unit and Z is a water-soluble group, in the extraction of copper (n=1 or 2 and x=0 or 1) by means of an aqueous leachant comprising sulfuric acid.
However, disadvantages of these known processes are the large amounts of sulfuric acid which are acquired to separate the copper from the copper-comprising materials and also the generally unsatisfactory yield of copper during the extraction, which is usually not more than 80%. Furthermore, the extraction takes up a considerable time ranging from a few days to a few months.